1. Field of the Invention
This invention relates to air springs, such as are used to load co-rotating rolls into nipping engagement on a papermaking machine. More specifically, this invention relates to an improved air spring having the capability of quickly releasing the air pressure and quickly exhausting the compressed volume of air within the air spring. Still more particularly, this invention relates to an air spring apparatus including a primary air spring having an enlarged exhaust port and a separate, secondary air spring which operates in conjunction with the primary air spring to control the opening and closing of its exhaust port to quickly shorten its effective stroke.
2. Description of the Prior Art
Air springs are well-known and are extensively utilized in many industries to provide resilient, or cushioned, load support and for providing substantial force to move or actuate components, such as pivoted lever arms, in machinery. Examples of the usage of air springs is in the suspension system on inter-city buses, long distance trucks, mining equipment, including vehicles, and in the roll actuation apparatus on papermaking machinery.
Specifically, in papermaking machinery, air springs are commonly used to actuate the lever arms used to bring rolls into co-running, nipping engagement to effect nip loads, for example, up to about 1,000 pli (pounds per linear inch) or about 178 Kg/cm. Air springs are versatile, powerful and have relatively short axial lengths. They are actuated by the application of air pressurized to relatively low pressures (60 psi/413,688 Pascals) which can be provided easily in most industrial, commercial and vehicular applications.
Even though the applications for air springs are almost endless, they have heretofore exhibited one major deficiency. Specifically, they have not been able to quickly collapse and quickly reduce their axial effective stroke or actuation distance. In some applications, this is not a troublesome characteristic, but in the case where they are used to load a roll couple, this operating characteristic can cause, or exacerbate, problems associated with the speed at which the nipped rolls are separated from one another to form a gap therebetween.
This operating deficiency has its basis in the fluid mechanics of pressurized air. When air is depressurized, the remaining volume does not inherently flow in the direction of the source of depressurization, such as a vent, without being forced out. Even under the force of the load, a relatively large volume of air will not pass through a relatively small orifice quickly. Thus, in prior air springs, when the exhaust port, which was usually a drilled hole, was opened, the air pressurization within the air spring was released virtually immediately, but the air spring would not collapse quickly, much less immediately, due to the fact that it remained substantially completely full of air. In other words, the movement of the air spring, and its opposed load plates, to a collapsed, or axially shortened position is not merely a function of the air pressure within the air spring, but of the volume of the air within the air spring in conjunction with the size of the exhaust port. The largest commonly available solenoid valves (for about a 5 cm diameter pipe) are inadequate.
Prior air springs were typically pressurized and exhausted through holes drilled in one of their outer plates. For unknown reasons, perhaps due to the need for continuous load bearing capability, such as in vehicular suspension systems, little or no capability was provided for air to be more rapidly introduced into the air spring and exhausted from it. However, as briefly mentioned above, in some applications in the papermaking industry, the necessity for quickly collapsing the air spring has been long sought. Indeed, in some applications, elaborate linkages have been devised to mechanically amplify both the relatively short stroke and to reduce the relatively long time required by the prior, standard air spring to collapse after the release of air pressure. Such apparatus is relatively complicated and expensive to manufacture, and do not represent a direct solution to the problem.